Sintered bearings excellent in quietness may be used as bearings for a small precision motor to be mounted on an information device such as a hard disk drive. Along with improvement in performance of the motor in recent years, there is a demand to increase a PV limit value of this type of sintered bearing up to PV>200 MPa·m/min. Further, it is also necessary to realize reduction in fluctuation of torque (improvement in initial running-in characteristics), improvement in durability (improvement in seizure resistance), improvement in quietness (improvement in acoustic absorption characteristics), and the like.
As the sintered bearing, an iron-based sintered bearing is publicly known. The iron-based sintered bearing is excellent in durability, but is disadvantageous in poor initial running-in characteristics and quietness. Therefore, a bronze-based sintered bearing is often used as the bearing for a small precision motor. As an example of the bronze-based sintered bearing impregnated with oil, for example, JP 2001-107162 A (Patent Literature 1) discloses a bronze-based sintered bearing containing Sn at 6 to 11 wt %, Fe and/or Ni at 1 to 5 wt %, and copper as the balance (claim 1).
Further, JP 3873275 B2 (Patent Literature 2) discloses a slide component obtained by using iron-based raw material powder and copper-based raw material powder that is flat powder, and by segregating copper on the surface side so that the ratio of copper is reduced and the ratio of iron is increased in a range of from the surface side toward the inner side (claim 1). Further, there is obtained a bearing by loading the copper-based raw material powder and the iron-based raw material powder into a loading unit, applying vibrations thereto so as to segregate the copper-based flat powder on the surface side and cover the surface side with copper, thereby attaining such a concentration gradient that the ratio of iron is higher than that of copper in a range of from the surface side toward the inner side (paragraph 0028).